The present invention relates to a half-loop antenna, particularly a half-loop antenna for use in a motor vehicle.
The half-loop antenna known from the literature includes a semicircular-shaped metallic curved conductor or curved antenna piece positioned over a ground plate (ground plane), as illustrated, by way of example, in FIG. 5. The mode of operation of the known half-loop antenna corresponds to that of a folded monopole antenna. Furthermore, its field pattern in the vertical and horizontal plane approximates that of a monopole, for example, that of a xcex/4 antenna. A half-loop antenna designed for a resonant length of xcex/2 has an overall height of 83% of that of a xcex/4 antenna. If one energizes one side of the curved conductor piece and contacts the other side to the base plate or earth plane, the antenna, at its resonant frequency has an impedance of 100xcexa9. Furthermore, increasing the capacity of an antenna effects a more broad-banded radiation behavior in the frequency band. Increasing the capacity of an antenna can also be reached effectively by enlarging the dimension of its voltage maximum. A xcex/2 half-loop antenna has its voltage maximum at half the antenna height, that is, at the highest point of the conductor half-loop above the ground plane.
An antenna unit is described in European Published Patent Application No. 0 684 661 which has a substrate, and an antenna fastened to the substrate, whose radiating portion is a flat plate arranged parallel to the substrate. The radiating portion has a feed terminal and a ground terminal.
Also, in German Published Patent Application No. 195 14 556 a flat antenna arrangement is described for frequencies in the GHz band, which includes an antenna for satellite-supported vehicle navigation (GPS) and at least one antenna for mobile radio, which are positioned in a common housing on a conducting surface of greater dimension, in particular, on a vehicle body. In this connection, the GPS antenna is preferably formed as a strip line antenna with broadside radiation, includes a plate made of a dielectric material, which is continuously metallized as ground surface on one side, and provided with partial metallization in the radiation direction on the other side, and wherein the mobile radio antenna has omnidirectional characteristics in the horizontal field pattern and the large conducting surface is used as ground reference surface for this antenna.
The disadvantage with the known flat antennas is their necessary area requirement, especially when they are used with motor vehicles.
Therefore, an object of the present invention is based on developing a half-loop antenna which can specifically be applied in the motor vehicle field for mobile radio, and wherein a type of construction can be achieved that is as compact and small in area as possible, while maintaining good antenna characteristics.
In a half-loop antenna according to the present invention, having a metallic curved antenna piece which is arranged opposite a ground plane designed as ground and the curved antenna piece is connected at one end with the ground plane and has the antenna signal at the other end, the curved antenna piece is formed by an area whose outer edge forms a convex curve, that is, it is arched toward the outside.
Preferably, the area of the curved antenna piece is arranged to be either parallel to the ground plane or arched toward the outside. The area of the curved antenna piece can also be arranged to be at an angle to the ground plane.
In a preferred embodiment, the development of the curved antenna piece has the form of an ellipse tapering to a point at its ends.
In order further to decrease the overall height of the antenna, an inductance is inserted at the antenna signal side of the curved antenna piece. In addition, the connection between the curved antenna piece and the ground plane can be made by a further inductance.
Preferably, the planar curved antenna piece has a dielectric on its outer side. Then too, the antenna can be protected by a radom, the radom being applied as dielectric.
In another preferred development, the inductance, or inductances, as the case may be, is/are formed as a spring, whose restoring force presses the metallic area of the curved antenna piece, or parts of it, against the radom.
The metallic curved antenna piece can also be applied to the inside of the radom as a metallic surface.
The antenna surface of the half-loop antenna can also be developed as a skeleton antenna, the surface of the curved antenna piece being formed by a thin metallic conductor which forms the outer edge of the antenna surface.
Advantageously, an increase in the capacity of the antenna at the smallest ground plane is effected by the design of the curved antenna piece as a surface with a convex edge, and this achieves a more broad-banded radiation behavior in the frequency band. Furthermore, by increasing the self-capacitance of the antenna, the impedance at the resonance or operating frequency can be shifted to lower values such as 50xcexa9. Advantageously, too, neither the horizontal nor the vertical field pattern is influenced by the selected geometry, or rather, is influenced to only a slight degree. Raising the capacity offers the possibility of shortening the mechanical length of the curved antenna piece, so that, with a corresponding shortening of the mechanical length of the curved antenna piece the overall height is reduced to 50% of a xcex/4 antenna.
It is especially advantageous that a feed network is positioned between the curved antenna piece and one of the antenna terminals, the feed network having at least one first resonant circuit which includes an inductance and a capacitance. In this way, the half-loop antenna can transmit or receive signals in at least two frequency ranges. Thus, a multiband-capable half-loop antenna is realized, which simultaneously, to the greatest extent possible, has a compact and small surface type of construction.
A further advantage is that the feed network includes at least one additional impedance, selected in such a way that the impedance of the half-loop antenna is adapted to a preset impedance at the antenna input. In this way, fine tuning of the impedance of the half-loop antenna in the respectively used frequency bands can be accomplished.
A further advantage is that the feed network has a plurality of resonant circuits of various resonant frequencies. In that way, more than two frequency ranges can be realized, in which the half-loop antenna can send and/or receive signals while simultaneously keeping its compact and small surface type of construction.